Abstract
High temperatures delay flowering in chrysanthemums. This heat delay disturbs stable planning production by light culture in summer. There was a marked difference in the degree of flowering delay caused by high temperature treatment among summer-to-autumn-flowering small-flowered spray-type chrysanthemum cultivars, which are suitable for open-field light culture. In the cultivars showing less delayed flowering caused by high temperature treatments, delay in flowering time also tended to be shorter in the hotter summer during three consecutive years of cultivation by light culture. In open-field light culture of summer-to-autumn-flowering small-flowered spray-type chrysanthemums, stable planning production with little annual fluctuation can be established using cultivars showing less heat delay. In addition, the expression level of the florigen gene (FTL3) promoting flowering was measured under high temperature conditions. In the cultivar showing a marked heat delay, the rise in the FTL3 expression level was strongly suppressed by high temperatures. On the other hand, the FTL3 expression level increased even under high temperatures in the cultivar showing less heat delay. The results suggest that suppression of the florigen gene by high temperatures is involved in the heat delay of flowering in summer-to-autumn-flowering small-flowered spray-type chrysanthemums, as is the case with autumn-flowering chrysanthemums and the wild chrysanthemum Chrysanthemum seticuspe.
